The power recovery string of a fluid catalytic cracker (FCC) process typically includes a hot gas expander or turbine, an axial air compressor, a motor/generator and a starting steam turbine. The machines are connected in axial alignment so that the power shaft of each serves as an extension of the other and they turn as a unit. In a fully operating FCC system, the hot products of combustion from a regenerator vessel are supplied to the hot gas expander for power recovery before being supplied to a boiler for the generation of steam. The hot gas expander drives the axial air compressor which compresses ambient air and supplies it to the regenerator vessel as combustion air for burning off the carbonaceous coating which forms on the catalyst. The hot gas expander also drives the motor/generator to supply electrical power to other parts of the plant or to the electric grid.
The starting steam generator is operatively coupled to the power recovery string for driving the axial compressor during start up of the power recovery string, when the hot gas expander is down for repairs or the like, and when there is insufficient hot gas for the expander. The motor/generator is also capable of driving the axial compressor either alone or in combination with the starting steam turbine and/or the hot gas expander. During a start up period, the hot gas expander is gradually brought up to operating temperature at a rate of 100.degree. to 200.degree. F./hour by gradually supplying hot regenerator gas thereto. When the hot gas flow to the expander increases to the point where it is able to overcome the parasitic losses which have an equivalent load, typically, of about 2000 horsepower, the expander assists the steam turbine and/or the motor/generator in driving the compressor. The axial compressor is inherently a constant volumetric device and therefore the load it provides is a function of the density of the ambient air being compressed and supplied to the regenerator vessel.
The total FCC system including the regenerator, separator, reactor and piping is dried out by passing hot gas therethrough. The catalyst is loaded prior to starting up the cracking operation.
When the FCC system components have been sufficiently dried, torch oil is introduced into the regenerator for additional dry out. The catalyst is then introduced and crude cracking begins. Gradually, the pressure is built up in the regenerator and the hot gasses are supplied in part to the hot gas expander. The rest of the hot gasses are bypassed to the waste heat boiler until the hot gas expander is fully operative. At that point, the hot gas expander will drive the compressor and the motor/generator will be driven as a generator. The starting steam turbine will free wheel so as to be able to drive the compressor if the hot gas expander goes off line or is running at reduced power.